The present invention relates generally to a froth flotation deinking (ink removal) process for use in paper recycling and, in particular, relates to a method and apparatus for monitoring the liquid and solid content in a froth as a function of froth conductivity.
The recycling of secondary fibers (for example wastepaper) is of growing importance in the protection of the environment by reducing the amount of solid wastepaper and sludge which is placed into landfills. Wastepaper is the largest contributor of the solid waste landfilled each year. About 52 million tons of wastepaper are currently landfilled and, thus, pollute the environment.
Although the paper recycling rate has increased in recent years, the quality of various grades of paper made from recycled wastepaper fibers is far poorer than the quality of similar grades of paper made with virgin (not recycled) paper fibers. Further, the cost of making paper from recycled paper fibers is significantly greater than the cost of making paper from virgin paper fibers.
The objective of paper recycling is to recover paper fibers from the wastepaper, such as photocopied documents or newspapers, which may contain ash, various chemicals, printed or copied inks (offset ink, copying toner particles, etc.) and/or other contaminants.
Froth flotation is an important technique used in the recycling of wastepaper for removing contaminants, such as ink, from the wastepaper. Froth flotation employs the principles of colloid chemistry and physics to separate floatable (hydrophobic) and non-floatable (hydrophilic) particles from each other in an aqueous slurry containing such materials based upon differences in the hydrophobicity of the materials.
Generally, a slurry made from the materials to be separated and water is mixed with 1) a dispersant that separates ink particles from the surface of wastepaper or wood fibers and to prevent the redeposition of separated ink particles on the fibers, 2) a collector that agglomerates small particles to large ones, and changes the surface of hydrophilic ink particles to hydrophobic, and 3) a frothing agent that generates a layer of foam at the upper portion of the froth flotation device for the removal of ink particles and other contaminants. A collector can also be added that causes a surface energy change between solid-liquid, solid-gas, and gas-liquid interfaces.
The mixture is introduced into a froth flotation device, and a pressurized nonreactive gas, such as air, is introduced into the bottom of the device by a generator, and is forced upwards into the slurry in the form of bubbles ranging generally in size from about 50 microns to about 10 cm in diameter. The air bubbles tend to attach to the hydrophobic particles present in the slurry, and cause those particles to rise upwards to the surface of the slurry as a froth layer. The carrying capacity of the bubbles is largely a function of the surface area of the bubbles per unit volume of the froth.
Froth flotation deinking processes thus typically involve three subprocesses: (a) detachment of the ink particles from the wastepaper or wood fibers; (b) adhesion of the ink particles onto air bubble surfaces; and (c) removal of froth and ink particles from flotation cells. The ink particles, many of which are hydrophobic, such as offset ink and copying toner particles, attach to the surface of the air bubbles and float upwards with the bubbles towards the upper portion of a froth flotation device during flotation. Under ideal conditions, the hydrophilic paper fibers will not attach to a hydrophobic air bubble surface. Accordingly, ink removal generally increases with an increase in the rate of froth removal (the rate at which froth is removed from the flotation device), and fiber loss should not occur. As used herein, a reject stream refers to the froth removed from a froth flotation deinking device, as is customary to one having ordinary skill in the art.
Unfortunately, in practice, a significant level of paper fiber loss can occur during froth removal, which mainly results from the physical entrapment (or water carry over) of the fibers in the air bubble network which rises towards the top of the froth flotation device to the froth layer. The mechanisms of pulp loss during froth flotation deinking are described in Ajersch and Pelton, “Mechanisms of Pulp Loss in Flotation Deinking,” J. Pulp and Paper Sci. 22, 9:J338-345 (1996). Fiber losses from 4 to 24% by weight have been observed, depending upon the conditions and equipment employed in the froth flotation deinking process. Such fiber loss significantly decreases paper recycling productivity, and correspondingly increases the costs of paper recycling. It is estimated that a 5% increase in the recovery of paper fibers during a froth flotation deinking process will achieve several advantages. First, the increase may significantly increase paper recycling productivity. Additionally, the increase may significantly reduce the costs of paper recycling. Furthermore, the increase may reduce the dry sludge production in a typical paper recycling mill. Accordingly, if a recycling mill has a capacity to recycle 250 tons of wastepaper per day, the recycled output could increase by about 2 tons per day (or 700 tons per year) from the 5% increase. A 10% increase in the paper recycling rate results in a reduction of 8.8 million tons of wastepaper in landfills each year. Even a 1% increase in the paper recycling rate results in a significant reduction of wastepaper in landfills each year.
It should be appreciated that online (real-time) measurement of fiber loss would enable the process control to decrease the entrapment of fibers in the froth. For instance, when a sudden increase of fiber loss is detected, the operator can adjust the flotation conditions to reduce froth stability or reduce the rejection rate momentarily to recover fiber yield. Furthermore, real-time measurement of fiber loss is a prerequisite for implementing any in-process control of fiber yield improvement technologies.
Fiber loss can be measured off-line after flotation deinking by gravimetric methods (i.e., by weighing the oven dry solid of the reject stream to determine the amount of total solid reject), then ashing the solid reject according to TAPPI standard methods to determine the fiber content in the dry solid (unburned materials and other inorganics in the paper are fillers, while the burnt materials are considered to be fiber).
Currently, no systems or methods are in place that are suitable for monitoring the solid content in the removed froth (also referred to herein as a reject stream) in real-time to enable an enhanced control of the dominating factors that affect fiber rejection during a deinking procedure.
What is therefore needed is a method and apparatus for the real-time monitoring of solid content in a reject stream during a froth flotation deinking procedure.